With the development of mobile communications systems, the mobile communications systems can provide increasingly higher transmission rates and better service quality for the users. The users, on the other hand, also have increasingly higher requirements on the transmission rates. To ensure the high rates for most users and to provide higher throughputs for certain users without significantly increasing the configured bandwidth, a carrier aggregation (CA) technology is introduced by the 3rd Generation Partnership Project (3GPP for short). Carrier aggregation means that a user equipment (UE) can use multiple component carriers (CC) simultaneously for both uplink and downlink communications, so as to support the high-speed data transmission. When the rate of a UE decreases, some component carriers may be released with only one resident carrier retained, and the released resources can be used by another user, thereby achieving an objective of flexible and dynamic configuration.
During the carrier aggregation, a primary cell (P-Cell) undertakes work such as security, handovers and bearing of a physical uplink control channel, another cell that participates in the aggregation is referred to as a member cell (Secondary Cell, S-Cell for short), and a member cell only undertakes data transmission work, which is not related to security or handover. For cell aggregation inside a base station, there is only one P-Cell, which bears physical uplink control channels of all aggregated cells. For cell aggregation among base stations, each base station uses its own primary cell. However, a UE management function such as handover management and security control can be undertaken only by a primary cell of one base station. Therefore, a primary cell of another base station is referred to as a primary secondary cell and marked as PCell*. Correspondingly, a base station (evolved NodeB, eNB for short) to which a PCell belongs is referred to as a primary base station (Primary evolved NodeB, PeNB for short), and another eNB to which a PCell* belongs is referred to as a secondary base station (SeNB). As a user equipment moves, the signal quality of the user equipment corresponding to cells also changes, and therefore signal quality of a PCell* is usually no longer reliable.